1. Field of the Invention
The present invention relates generally to the use of solar and thermal energy and more particularly to the conversion of solar and thermal energy to electrical energy.
2. Description of the Related Art
The conversion of solar energy to electrical energy through the use of photovoltaic cells is well established in the art. Photovoltaic cells are semiconductor components that convert light into useable electrical energy. A typical photovoltaic cell, commonly referred to as a solar cell, is comprised of an interface between an n-type semiconductor material and a p-type semiconductor material. A thin transparent layer of n-type or p-type material is deposited on a p-type or n-type material respectively to form an active p-n or n-p junction. When the junction is exposed to visible or nearly visible light, in a solar cell application, electron hole pairs, or minority charge carriers, are created at the junction. The minority charge carriers at the n-p interface migrate across the junction in opposite directions producing an electrical potential or an electrical voltage difference. In solar cell applications, electrical contacts, sometimes called ohmic contacts, are connected to the n-type and p-type materials on either side of the junction and an ensuing electric current is obtained.
The prior art has disclosed many variations of the basic p-n junction interface. Many of these variations have been attempts to improve the efficiency and effectiveness of the solar cell at absorbing solar energy. For example, a heterojunction photovoltaic device is comprised of stacked p-n junctions of different materials with band gap energies that match different parts of the solar spectrum. U.S. Pat. No. 4,332,974 discloses a multi layer photovoltaic cell wherein the first p-n layer will absorb energy in a particular band of the spectrum while the remaining energy passes through to the next p-n layer. The next subsequent p-n layer in the stack is comprised of materials that absorb a different band of the spectrum from the preceding layer. Each preceding layer acts as a window to the remaining energy of the spectrum that it does not absorb. With the cells arranged in such a fashion, the amount of solar energy converted to electrical energy is expanded, thus increasing the efficiency of the device.
Another example of a prior art variation of the basic p-n junction is the p-I-n junction. The p-I-n junction is comprised of p-type semiconductor material and n-type semiconductor material separated by an intrinsic-type material semiconductor material. The addition of the intrinsic-type material layer creates a diffusion potential between this layer and the p-type material and the n-type material. The p-I-n device is constructed such that the majority of the incident light energy is absorbed in the intrinsic layer allowing more of the positive and negative charge carriers to diffuse toward their respective p-type and n-type interfaces. This variation on the basic p-n junction enhances the flow of the charge carriers and improves the overall efficiency and effectiveness of the photovoltaic cell.
Typically, the individual interfaces of photovoltaic cells are interconnected to form an array or panel to supply electrical power. Regardless of the type of junction, the photovoltaic cells and the resulting arrays are subsequently interconnected in series/parallel connections to supply the required voltage and current output.
There are many cases of prior art wherein photovoltaic cells are enhanced to increase efficiency of a solar panel. For example, U.S. Pat. Nos. 4,002,499, 4,003,638, 4,088,116, 4,129,115, and 4,312,330 all disclose various methods of concentrating the incident light energy entering a photovoltaic cell. The common theme among the above cited examples is the use of a reflective device to collect sunlight distributed over a larger area and focus it upon a photovoltaic cell thereby increasing the amount of incident light energy.
The use of solar panels to convert light energy into thermal energy is also well known in the art. There are many examples of prior art which utilize light energy to passively heat fluid. For instance, U.S. Pat. No. 5,522,944 discloses the use of interconnected tubes disposed within an array of photovoltaic cells for converting solar energy to thermal energy in a fluid disposed within the tubes.
Likewise, the use of a thermoelectric generator to convert thermal energy into electric energy is well known in the art. Thermoelectric generators are semiconductor or solid state devices which convert thermal energy to electrical energy directly. Unlike photovoltaic cells however they are restricted to a maximum possible thermal efficiency of 1-(T.sub.L /T.sub.H). This relationship is referred to as the Carnot efficiency and is calculated at the operating temperature between the source temperature, T.sub.H, and the sink temperature, T.sub.L.
Thermoelectric generators can be analyzed by using simple thermodynamic relationships at the macroscopic level unlike photovoltaic cells which normally require extensive analysis at the microscopic level. Simple fundamental relationships are utilized in the area of art to aid in understanding the function of the solid state devices employed in thermoelectric generators.
Thermoelectric generators are based on the Seebeck effect which holds that when two dissimilar materials are exposed to a temperature differential an electric current will be generated at their junction. The suitability of the materials for the thermoelectric device depends primarily on a parameter referred to as the figure of merit. The figure of merit is based on the material type evaluated at the perceived operating temperature of the thermoelectric device. The higher the value of the figure of merit in the temperature range of the thermoelectric device the better suited the materials are for a thermoelectric device. It is well known in the art to optimize the figure of merit for candidate materials by optimizing material geometries along with material types. In order to optimize the figure of merit an area ratio between the n-type and the p-type materials is selected such that the following relationships are satisfied: ##EQU1## and EQU 1.sub.n =1.sub.p
where
A.sub.n area of n-type material PA1 A.sub.p area of p-type material PA1 .rho..sub.p, .rho..sub.n electrical resistivity PA1 .lambda..sub.p, .lambda..sub.n thermal conductivity PA1 1.sub.p, 1.sub.n Length of area elements.
With the semiconductor materials selected based on the above equations, the figure of merit, Z, is optimized by satisfying the following relationship:
where ##EQU2## .alpha..sub.p, .alpha..sub.n Seebeck coefficients.
For the optimum figure of merit, Z, the optimum current, I.sub.opt, produced by the thermoelectric generator is calculated by the following equation: ##EQU3## where ##EQU4## and T.sub.H, T.sub.L are the high and low temperatures of the source and the sink, respectively.
and EQU .chi.=[1+Z((T.sub.H +T.sub.L)/2)].sup.1/2
The open circuit voltage for the thermoelectric generator, .sub.Voc, is calculated by the following equation: EQU V.sub.oc =(.vertline..alpha..sub.p .vertline.+.vertline..alpha..sub.n .vertline.)(T.sub.H -T.sub.L)
The specific thermal efficiency of the thermoelectric generator for the optimized conditions then becomes: ##EQU5## Note that it is not possible for the thermoelectric generator to have a thermal efficiency greater than the previously stated Camot efficiency and as such T.sub.L /T.sub.H at the operating conditions of the device must be less than one.
An example of a thermoelectric generator is disclosed in U.S. Pat. No. 4,338,560. The thermoelectric generator of the '560 patent discloses a generator that comprises an array of sources and sinks interconnected by n-type and p-type doped material elements. It is disclosed that the sources absorb infrared heat from the earth and the sinks emit excess heat to space.
State of the art photovoltaic cells work well during daylight hours or when there is a sufficient incident light source, while thermoelectric generators tend to work better at night. What is needed is a thermoelectric-photovoltaic cell system with both enhanced terrestrial and space capabilities which employs state of the art design and manufacturing techniques to obtain maximum electrical energy output from the solar cells during daylight and sunlight conditions and from thermoelectric generator cells from temperature differentials.